Sectoring is a well known technique for providing distinct coverage areas within individual cell sites and can be achieved with “smart antenna” technology. Smart antenna methods dynamically change the radiation pattern of an antenna to form a “beam,” which specifically focuses the antenna's transmitted and received energy and provides a desired topographical coverage. Beam forming is an enhancement on sectoring in that the sectors can be adjusted in direction and width. Both techniques are employed to: 1) reduce interference between cells and the wireless transmit/receive units (WTRUs) deployed within the cells; 2) increase the permissible range between a receiver and a transmitter; and 3) locate the geographic position of a WTRU. These techniques are usually applied to the dedicated channels of the WTRUs once their general location is known.
Prior to knowing the location of a WTRU, the common channels broadcast information that all WTRUs may receive. While this information may be sent in static sectors, it is not sent in variable beams. There are inherent inefficiencies in this approach in that extra steps are required to determine the appropriate beam to use for the dedicated data exchanges. Additionally, the beams must be generally large enough to provide a broad coverage area, which in turn means their power decreases with distance from the transmitter.
The common channel coverage found in the prior art shown in FIG. 1 has four overlapping wide beams. This provides omni-directional coverage, while giving a degree of reuse to the cell site. It also provides a coarse degree of directivity to the WTRUs (WTRU1, WTRU2) detecting one of the transmissions, by having each sector transmit a unique identifier.
Referring to FIG. 2, downlink dedicated beams between a primary station (P) and several WTRUs (WTRU3, WTRU4) are shown. Assuming the same power from the primary station P for FIGS. 1 and 2 and all other attributes being equal, the WTRUs (WTRU3 and WTRU4) shown in FIG. 2 can be further away from the primary station P than the WTRUs (WTRU1, WTRU2) shown in FIG. 1. Alternatively, the coverage areas can be made approximately the same by decreasing the symbol rate or increasing the error correction coding. Either of these approaches decreases the data delivery rate. This also applies to the receiver uplink beam patterns of the primary station P; and the same comments about coverage and options apply for data from the WTRUs to the primary station P.
In the prior art, the range of a primary station P or a WTRU is generally increased by combinations of higher power, lower symbol rates, error correction coding and diversity in time, frequency or space. However, these methods yield results that fall short of optimized operation. Additionally, there is a mismatch between the common and dedicated communications channels in the ways that coverage is aligned.
The downlink dedicated channels may be transmitted in a beam having a narrower width by a smart antenna. The narrower beam serves a narrower area. The benefit in narrowing the beam is the reduced interference to WTRUs in other areas of the cell, which has a positive impact on the system efficiency. However, dedicated channels are still susceptible to interference generated by the common channels. The common channels have to be available to all mobiles in the entire coverage area. FIG. 3 shows the radiation pattern for the current deployment of a cellular system using a smart antenna system emitting a beam with a narrow width over a small coverage area 10 for the dedicated channel coverage and an omni-directional antenna emitting an omni-directional pattern over a wide coverage area 12 for the common channel coverage. Since the common channel is transmitted at a high output power to ensure complete cell coverage, a WTRU's reception of the dedicated channel may be interfered with as the WTRU's location becomes closer to the high powered common channel transmitter.
It is therefore desirable to provide a method of providing equitable coverage for both common and dedicated channels in wireless communication systems without the disadvantages of prior art.